The Canvas inbox feature is great for reaching many students at once unless you have a Canvas group that combines students from multiple sections of the same course. The inbox feature allows you to easily find and message individual members of the group. As an administrator of the group, there is no "message all members of the group" option when composing a new message.
It would be great to have the ability (at least as a group administrator) to message all members of the group. Currently the group administrator only has the power to change the name of the group.